The invention relates to an exhaust turbocharger for an internal combustion engine with a turbine and a compressor driven by the turbine and including means for controlling the flow volume delivered by the compressor.
The operating range of compressors in exhaust turbochargers is limited by a surge line and a choke line. These lines denote boundary lines in performance graphs that characterize the behavior of the compressor. When the compressor is operated close to the surge line, there are locally limited separation zones at the blades of the compressor impeller, and these can lead to pulsations of the flow with a periodic reversal of the direction of flow. The surge line of the compressor can be reached when relatively small volume flows are being delivered by the compressor.
When the operating point of the compressor shifts towards large volume flows in the compressor""s performance characteristics, however, the choke line may be reached. This is distinguished by blockage of the volume flow at the entrance to the impeller.
In order to extend the operating range of compressors, measures can be taken to stabilize the performance characteristics. These measures shift both the surge line and the choke line towards an extended operating range. One such measure to stabilize the performance characteristics is known, for example, from the publication DE 42 13 047 A1, according to which a bypass is provided in the compressor housing parallel to the compressor inflow duct to bypass one section of the inflow duct. If the compressor is operated in the region of the surge line, the bypass allows controlled recirculation of a mass flow component, which is recirculated in the bypass counter to the delivery direction. The mass flow component is fed back to the main mass flow and returned to the compressor together with the latter.
The choke line too can be shifted to give an extended operating range by stabilizing the performance characteristics. As the volume flows increase, intake air is fed into the compressor because of the additional bypass flow cross-section, thereby shifting the choke line in the direction of larger mass flows.
The disadvantage of the arrangement as shown in DE 42 13 047 A1 is that the measure to stabilize the performance characteristic is integrated in a fixed and invariable manner into the housing of the compressor, without any possibility of adjustment. It is therefore impossible to adapt the flow in the bypass (individually) as a function of the operating point of the compressor.
DE 198 23 274 C1 represents an improvement in this respect. In the compressor described in this publication, an adjustable closure element is provided for variable adjustment of the effective flow cross-section of the bypass. This element is an adjustable guide vane structure comprising two concentrically arranged rings.
It is the object of the present invention to provide, by simple means, a compressor in an exhaust turbocharger for an internal combustion engine with extended performance characteristics.
In an exhaust turbocharger for an internal combustion engine including a turbine and a compressor which is driven by the turbine, wherein the compressor comprises a compressor impeller in a flow duct, at least one transfer duct extends radially around the compressor impeller in the axial direction of the impeller and provides for communication with the flow duct. To provide for variable adjustment of the effective flow cross-section of the transfer duct, a transfer ring with transfer grooves is provided which comprises two component rings, which are rotatable relative to one another.
Preferably, at least one of the two component rings can be angularly adjusted by means of an actuating element depending on the operating condition of the internal combustion engine. In the overlapping position, the free cross-section of the transfer slot becomes a maximum whereas, in the blocking position, the free flow cross-section is reduced to a minimum, if appropriate to zero. With the component rings being adjustable rotationally relative to one another, the free flow cross-section through the transfer slots at the interface between the two component rings can be adjusted.
The two component rings are advantageously arranged concentrically and radially one within the other. The radially inner component ring is disposed adjacent to the compressor impeller and the radially outer component ring is disposed remote from the compressor impeller. In the overlapping position, the cross-sections of the transfer slots on the radially inner side of the outer component ring overlap with those of the inner ring. The blocking position is achieved by rotation of the two component rings relative to one another, with the two corresponding cross-sections of the associated transfer slots in the inner and outer component rings moving into a position out of radial alignment. In the blocking position, the transfer duct is thus formed solely by the remaining open transfer slot of the inner component ring, which preferably extends radially through the entire wall of the inner component ring.
The free flow cross-section of the transfer slot is substantially determined by the depth of the transfer slot measured in the radial direction. The depth of the transfer slot of the inner component ring is preferably between 10% and 50% of the total depth of both transfer slots, formed by addition of the individual slot depths of the inner component ring and the outer component ring. The maximum possible volume flow through the transfer slot in the blocking position can accordingly be reduced to 10% to 50% of the amount in the overlapping position. Reduction of the free flow cross-section may be considered especially in the operating range between the surge line and the choke line of the compressor performance graph since little or no air recirculation should take place in this stable region of the performance graph so as to avoid impairing optimum air delivery via the compressor impeller.
A plurality of flow slots is advantageously distributed uniformly over the circumference of the two component rings in order for the air flow through the flow slots to be uniform over the circumference. The flow slots may not arranged radially in the component rings but at a particular angle relative to the circumferential direction; they preferably extend tangentially in the component rings. This imparts a desired rotational component to the airflow.
As an alternative or in addition to component rings situated radially one above the other, it may also be appropriate to provide two or more component rings axially one adjacent the other. The flow slot also can be adjusted by rotation of one or more component rings into an overlapping position or into a blocking position. In this embodiment, it is possible, in particular, to manipulate the axial extent of the flow slots. The axially successive component rings can also be combined, if appropriate, with component rings that lie radially one above the other. Finally, it is also possible to arrange more than two component rings radially one above the other and/or axially one behind the other.
Advantages embodiments of the invention will be described below on the basis of the accompanying drawings.